Ion detector, detector array and instrument using same

ABSTRACT

An array of ion detectors ( 30 ) comprising a plurality of pickup electrodes ( 20, 34 ) for receiving ions; a substrate ( 32 ); a plurality of insulators ( 22, 35 ) positioned respectively between said pickup electrodes ( 20, 34 ) and said substrate ( 32 ); a plurality of charge storage areas ( 12, 38 ) for storing charge received by said pickup electrodes ( 20, 34 ), wherein each area ( 12, 38 ) is connected to a particular pickup electrode ( 20, 34 ) and means ( 44 ) for determining the amount of charge collected by each charge storage area.

This application claims benefit to Provisional Application 60/015,303filed Apr. 12, 1996.

TECHNICAL FIELD

This invention relates to low noise solid state charge integratingdetectors. More particularly, it relates to single channel ion andelectron detectors and to ion and electron measuring array detectors.

BACKGROUND ART

Mass spectroscopy is just one of several analytical techniques whichrequire ion or charged particle detectors. Other applications in whichion or charged particle detection is required include electron energyanalyzers, electron capture detectors, flame ionization detectors,photoionization detectors, ion mobility spectrometers, smoke andparticle detectors or any application in which ions in solutions arecollected and measured. Typically in applications which require an arrayit is necessary to use a costly and complex micro channel plate,phosphor-fiber optic-photodiode array assembly to detect ions directly.In single channel applications it is possible to detect ions directlywith a multiplier device such as an electron multiplier, a channelelectron multiplier (CEM) or a discrete dynode electron multiplier. Itis also possible to use a phosphor to convert ions to photons, and thendetect them with a photomultiplier. A Faraday cup collector and anelectometer may also be used.

Replacement of channel electron multipliers or other detectors in, forexample, quadrupole mass spectroscopy and in other applications would beof value in providing cost savings and improved performance. Preferablya detector should be insensitive to vacuum quality and should not beadversely affected by exposure to atmosphere. Further, if at allpossible, it should not require high voltages, should not exhibit massdiscrimination, and should not respond to neutral particles or lowenergy photons.

U.S. Pat. No. 5,386,115 to Freidhoff et al. discloses a solid state massspectrograph which includes an inlet, a gas ionizer, a mass filter and adetector array all formed within a cavity in a semiconductor substrate.The detector array is a linear array oriented in the dispersion plane ofthe mass filter and includes converging electrodes at the end of thecavity serving as Faraday cages which pass charge to signal generatorssuch as charge coupled devices formed in the substrate but removed fromthe cavity.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a charge sensitive detectorthat exhibits high sensitivity and dynamic range and which isinsensitive to the gas pressure in the detector's environment.

In accordance with a first embodiment of the invention, high sensitivityand dynamic range can be achieved that are comparable to thoseachievable with electron multipliers, but without the incorporation ofsuch a multiplier. Hence, in many applications that utilizes such amultiplier for signal amplification, the multiplier can be replaced witha much more cost effective charge sensitive detector in accordance withthe invention without suffering significant loss in performance, whilealso reducing the requirement for high vacuum that may have been imposedby the use of the multiplier. The use of the detector in accordance withthe invention also removes the requirement for high voltage that isotherwise required for operation of the electron multiplier. Further,and perhaps more significantly, such performance is achievable inenvironments where poor vacuum, or high gas pressures, exists, i.e.,where electron multipliers can not be used due to their inherentrequirement for a good vacuum environment. Hence, much highersensitivities can be achieved in such high pressure environments thanwas previously achievable. Further, charged particles of much highermass can be detected. In addition a detector according to the inventionis insensitive to neutral atoms or molecules.

Using an integrated chip metallization for the pickup electrode, if ofthe order of 10 mm in diameter, would result in very high electrodecapacitance, even with the thickest dielectric layers available indevice manufacture. This high capacitance would make it impossible toachieve very low read noise levels.

A way to avoid this problem is to use an isolated pickup electrode,which may be made in any number of ways: machined metal, punched orelectroformed metal, molded conductive material, conductively platedmolded material, vapor deposited material, etc. An important aspect ofthe invention is to support the pickup electrode at sufficient distancefrom surrounding conductors, such as the device substrate, to reduce theelectrode capacitance to a low value of, typically 1 pF or less. It isalso necessary that the supporting structures have extremely lowconductance, typically 10¹³ ohms, and that the dielectric constants andgeometry of the supports be consistent with the low electrodecapacitance to the surroundings.

If this is done, other problems arise. At the very low detection limitsrequired, such structures will tend to be extremely sensitive to (a)microphonics and (b) stray electrostatic fields that are time-varying.

Microphonics, or induced voltage variations on the pickup electrode dueto mechanical vibrations of the electrode or surrounding conductionstructures, can be reduced or eliminated by (1) making the electrode andall surrounding structures very rigid and (2) by arranging for there tobe no net charge on the pickup electrode. The vibration induced voltageon the electrode is proportional both to the vibration inducedcapacitance variation between the electrode and its surroundings, and tothe charge on the electrode. By surrounding the electrode with a“Faraday Cage” biased to the potential of the pickup electrode, chargeon the electrode is minimized. There are at least 2 options here: (1)Put a fixed bias on the Cage, or shield, which is nominally equal to thereset potential of the pickup electrode. This will result in immunityfrom microphonics at zero or very small ion currents, but as the signalincreases, the electrode will acquire some charge toward the end of eachintegration cycle, and will be subject to microphonic noise. However, itis precisely at low signal levels that the lowest noise is required, sothis is generally acceptable. Option (2) is to bootstrap the FaradayCage potential to that of the electrode, tracking it during eachintegration. The effect is essentially to eliminate the effectivecapacitance of the pickup electrode, so that all charge accumulation isin the MOS circuit itself. This option is more complex, but offerspotentially better performance and, if done carefully, permits largerdetector areas to be realized.

The influence of stray AC fields can also be controlled by placing thepickup electrode within a cage, with apertures or grids provided toallow entrance of the ion flux which is to be measured. If the fieldsare very large, multiple layers of shielding may be required. The meanpotential on the cage must be held within 1 mV or better of the optimumvalue, and AC components at frequencies which would interfere with thedetection process must be held to microvolt levels or less.

It is also vital that any electrical leakage paths from the pickupelectrode to any other conducting surfaces be minimized. In particular,the RC time constants should be kept 2-3 orders of magnitude higher thanthe desired integration cycle times, depending on the desired chargemeasuring accuracy. Small amounts of leakage may be dynamicallycalibrated away. In particular, any contamination of surfaces bymaterial in the sample or sample beam, or sputtered from the detectorassembly or elsewhere by sample ions, must not be allowed to formconductive paths. This goal can be achieved by appropriate geometricaldesign which physically shields critical surfaces from directcontamination.

In a second embodiment of the invention, charge supplied by an ion orelectron current in vacuum or gas is deposited on an integratedelectrode, typically of dimensions 10 um by 1000 um. This electrode isconnected to a MOSFET circuit capable of resetting the voltage on theelectrode to a preset value and then reading out the charge (voltage) onthe electrode after a set integration time. The capacitance of theelectrode and its associated circuit (FET gate, etc.) is typically 0.5to 1 pF. A read noise level as low as several electrons is achieved.Alternatively CCD circuits can be used to detect the charge and providesignals indicative of the amount of charge present on the electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged perspective view of a single channel detector inaccordance with the invention.

FIG. 2 is a further enlarged cross-sectional view of the detector ofFIG. 1.

FIG. 3 is an enlarged perspective view of an array detector inaccordance with the invention.

FIG. 3A is a further enlarged portion of the array detector of FIG. 3.

FIG. 4 is an enlarged schematic cross sectional view of the array ofFIG. 3.

FIG. 5 is a simplified schematic diagram of the array of FIG. 3.

FIG. 6 is a diagram of the manner in which a large array may beconstructed from several small arrays.

FIG. 7 is a block diagram of a spectrometer using an array detector inaccordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows schematically a first embodiment of the invention in theform of a charge integrating detector chip shown generally as 10 formedon a silicon substrate 11 with an integrated charge detection circuit 12having a small exposed bonding signal pad 14 for signal pickup. Thereare also a number of normal bonding pads 16 for external connections tothe chip. A large area metallization region, or other conductivesurface, is provided as the bottom part of a Faraday shield. It may beinterrupted by the circuit traces, or it may be a second metallizationcovering substantially all the chip area except for the signal pad 14. Apickup electrode 20 is shown suspended above the chip, with a bondingwire 22 attaching it to the signal node. Supports for the pickupelectrode 20 and a top shield are not shown in FIG. 1.

The pickup electrode 20 may be in the form of a flat plate of any shape,a cup, or of other suitable form. To reduce scattering and secondaryelectron emission, it may have a multiwell configuration, such as ahoneycomb, so that ions are actually trapped within deep columnarstructures and any scattered or secondary ions will require multiplebounces to escape.

FIG. 2 shows a cross section of the complete detector, including theelectrode supports 22 and the top shield 24, which is electricallyconnected to the bottom shield metallization region 18, which, asindicated previously, is electrically biased to a potential very nearthe signal node potential (the potential of pad 14). Electrode supports22 may be of many forms: The electrode 20 may be supported by one ormore globs of low dielectric constant, highly insulating polymer orother material, such as an open cell foam adhesive. Alternatively,electrode 20 may be supported by three or more low cross section soliddielectric legs or supports, or by a sufficiently thick (many mm) soliddielectric layer. The dimensions shown, an 8 mm diameter pickup located2 mm from both the top and bottom of the Faraday Cage, result in apickup capacitance of approximately 0.5 pF. The dimensions can beadjusted to achieve lower capacitance, or similar capacitance withlarger pickup area.

Referring to FIG. 3, the principles of the present invention are appliedto a second embodiment of the invention in the form of an integratedarray detector shown generally as 30. Array detector 30 is formed on asilicon substrate 32. The array includes numerous detector elements 34which may be formed of aluminum, or other conductive material, or byreactive ion etching of a more refractory material such as tungstenwhich is resistant to sputtering. As may be seen in FIG. 3A, eachdetector element 34 is connected by an extension 36 of its metalizationto a charge storage circuit 38. The array elements may be of anyshape,and spaced uniformly or non-uniformly. Charge storage circuit 38may be a MOSFET circuit or a CCD circuit as described above.

The output of each charge storage circuit 38 is connected to an outputline 40. Lines 40 form a signal bus 41. The voltage output of each line40 is multiplexed with a multiplexer circuit (not shown in FIG. 3) to ananalog to digital converter 42 and eventually sent to a computer 44which acquires, stores and analyzes the data obtained by array detector30.

Referring to FIG. 4, each detector element 34 has an insulating material35 under it and therefore disposed between detector element 34 andsubstrate 32 to prevent voltage breakthrough and to minimize thecapacitance to substrate 32. Insulating material 35 may be, for example,a 3 μm thick layer of silicon dioxide. Other thicknesses or materialssuch as silicon nitride or a polyimide may also be used, alone or incombination.

Each detector element 34 may have a length of two millimeters, be spaced12.5 μm center to center with a 2 μm gap between adjacent elements 34. Atypical array may include 1,024 electrodes. However, smaller or largernumbers may be present and an array may include 4,000 or more detectorelements.

Each charged storage circuit 38 may be operated in a quadruplecorrelated sampling (QCS) or double correlated double sampling (DCDS)mode. Thus, the stored voltage is measured before sampling when nosignal has been accumulated and after sampling. Each measurement isfurther compared to a first reference (the reset voltage) to eliminateboth thermodynamic (kTC)^(½) reset noise and 1/f amplifier noise. Thus,four measurements are taken:

(1) While reset switch is on (clamped to bias reference)

(2) After reset

(3) At end of integration

(4) During reset clamp

The difference between measurements (1) and (2) is the kTC reset noiseq_(v). The difference between measurements (3) and (4) is the finalcharge Q_(F).

Thus, the detected charge is:

Q_(D)=Q_(F)−Q_(N)=(3)−(4)−[(2)−(1)]

Measurement (4) can be used as measurement (1) for next integrationinterval.

Referring to FIG. 5, a multiplexing circuit 48 connected to signal bus41 is used to successively read out the voltages stored on chargestorage circuits 38 by supplying the output voltage to A to D converter42.

Referring to FIG. 6, a plurality of array detectors 30A, 30B, 30C and30D are tiled together to form an extended array. Each detector is shownin plan view and the beam of ions is perpendicular to the plane of thefigure. The ion beam is assumed to be at least twice as wide as thedetectors so that one half of the beam impinges on detectors 30A and 30Cwhile the other half impinges on detectors 30B and 30D. This results ina reduction in sensitivity by a factor of approximately two. However,this arrangement avoids the “black line” problem of having spaces in theextended array so that when it is used as a detector in an image planeapplication there are locations at which no charged particles aredetected.

Referring to FIG. 7, a Mattauch-Herzog mass spectroscopy system (whichmay be combined with a gas chromagraph) utilizes a detector 50 inaccordance with the invention placed in an image plane 51. The signalsfrom the elements of detector 50 are provided to an image readoutcircuit 52 analogous to the analog to digital converter circuit 42described above. A computer 54 is analogous to computer 44 previouslydescribed, but may also be used to control other functions within thespectrometer such as the Z-lens voltage circuit 56.

As is well known in the art, computer 54 may also control an ion sourcevoltage circuit 56 and an electrostatic analyzer voltage circuit 58. Ina mass spectrometer the electrostatic analyzer provides a chargedparticle beam of relatively constant energy so that subsequent sortingby momentum will translate into sorting by mass as energy is heldsubstantially constant.

Other signals may be exchanged between computer 54 and a mechanicalroughing pump 60 which backs a turbomolecular pump 62 which alsointerchanges signals with computer 54. However, if the detector 50 inaccordance with the invention is used, in many applicationsturbomolecular pump 62 will not be necessary.

Pump 60 (and possibly turbomolecular pump 62) evacuate a chamber 64which includes an ion source 66. The ions may be those eluted from thecolumn of a gas chromatograph 68. These ions pass through an object slit70 and form an ion beam shown generally as 72. Beam 72 passes through anelectric sector 74 and a Z focus lens 76 before entering a magneticsector 78. In magnetic sector 78 the ions are dispersed according to thesquare root of their mass thus producing a spectrum of mass versusposition in the image plane 51.

The detector 50 of the present invention has a dynamic range ofapproximately six orders of magnitude. Greater dynamic range can beachieved by modulation of the ion current or by changing the rate ofreadout which may be in the order of 100 times per second, but can bevaried depending upon the application. Further, the detector accordingto the invention is sensitive to a mass range of 1 to at least 1,000atomic mass units, but in principle the mass range may be especiallyunlimited.

Various engineering considerations will occur to those skilled in theart. For example, those portions of the array associated with on-chipcharge storage transfer, applification and digitization should beshielded from ion and photon bombardment. This can be accomplished bysuitable passivation and metalization coatings or external shields.Further, charge buildup between electrodes and other metalizations canbe minimized by the use of suitable guard rings. Finally an array inaccordance with the invention can be mounted in an integrated circuitchip package for ease of handling or onto a custom purpose package forease of positioning in the image plane 51.

We claim:
 1. An array of ion detectors comprising: a plurality of pickupelectrodes for receiving ions; a substrate; a plurality of insulatorspositioned respectively between said pickup electrodes and saidsubstrate; a plurality of charge storage areas for storing chargereceived by said pickup electrodes, wherein each area is connected to aparticular pickup electrode; and means for determining the amount ofcharge collected by each charge storage area.
 2. The apparatus accordingto claim 1 further comprising a multiplexer for providing a multiplexedoutput indicative of the amount of charge in said charge storage areas.3. The apparatus according to claim 1 as used in a mass spectrometerincluding an ion source, an ion separator and a vacuum means.
 4. Theapparatus according to claim 3, wherein the vacuum means consists of aroughing pump.
 5. An ion detector comprising: a pickup electrode forreceiving ions; a voltage plane; a shield in close electrical contactwith said voltage plane such that the combination of said shield andsaid voltage plane form a Faraday cage; means for electrically biasingsaid Faraday cage; means for supporting said pickup electrode withinsaid Faraday cage to achieve a desired spatial relationship andcapacitance between said pickup electrode and said Faraday cage; chargedetection circuitry having an input and an output, said charge detectioncircuitry for detecting said ions received by said pickup electrode;means for connecting said pickup electrode to said charge detectioncircuitry input; means for producing a pickup electrode referencevoltage; and means for applying said pickup electrode reference voltageto said pickup electrode.
 6. The apparatus according to claims 5 whereinsaid shield is provided with apertures or grids to allow ions to impingeon to said pickup electrode.
 7. The apparatus according to claims 5wherein said means for electrically biasing said Faraday cage furthercomprises means for maintaining said Faraday cage at said pickupelectrode reference voltage.
 8. The apparatus according to claim 5wherein said means for electrically biasing said Faraday cage furthercomprises means for maintaining said Faraday cage at the same potentialas said pickup electrode.
 9. The apparatus according to claim 5 whereinsaid means for supporting said pickup electrode within said Faraday cageachieves a capacitance between said pickup electrode and said Faradaycage of less than 1 pf.
 10. A mass spectrometer comprised of at least anion source, an ion separator and an ion detector wherein said iondetector comprises: a pickup electrode for receiving ions; a voltageplane; a shield in close electrical contact with said voltage plane suchthat the combination of said shield and said voltage plane form aFaraday cage; means for electrically biasing said Faraday cage; meansfor supporting said pickup electrode within said Faraday cage to achievea desired spatial relationship and capacitance between said pickupelectrode and said Faraday cage; charge detection circuitry having aninput and an output, said charge detection circuitry for detecting saidions received by said pickup electrode; means for connecting said pickupelectrode to said charge detection circuitry input; means for producinga pickup electrode reference voltage; and means for applying said pickupelectrode reference voltage to said pickup electrode.
 11. The apparatusaccording to claim 10 wherein said shield is provided with apertures orgrids to allow ions to impinge on to said pickup electrode.
 12. Theapparatus according to claim 10 wherein said means for electricallybiasing said Faraday cage further comprises means for maintaining saidFaraday cage at said pickup electrode reference voltage.
 13. Theapparatus according to claim 10 wherein said means for electricallybiasing said Faraday cage further comprises means for maintaining saidFaraday cage at the same potential as said pickup electrode.
 14. Theapparatus according to claim 10 wherein said means for supporting saidpickup electrode within said Faraday cage achieves a capacitance betweensaid pickup electrode and said Faraday cage of less than 1 pf.
 15. Amethod of detecting ions comprising: forming a Faraday cage from acombination of a voltage plane and a shield, said voltage plane and saidshield being in close electrical contact; electrically biasing saidFaraday cage; supporting a pickup electrode within said Faraday cage toachieve a desired spatial relationship and capacitance between saidpickup electrode and said Faraday cage; producing a pickup electrodereference voltage; firstly applying said pickup electrode referencevoltage to said pickup electrode; measuring the pickup electrodepotential while said pickup electrode reference voltage is firstlyapplied; disconnecting said pickup electrode reference voltage from saidpickup electrode measuring the pickup electrode potential immediatelyafter disconnecting said pickup electrode reference voltage from saidpickup electrode allowing electrons to impinge on to said pickupelectrode measuring the pickup electrode potential immediately afterallowing electrons to impinge on to said pickup electrode secondlyapplying said pickup electrode reference voltage to said pickupelectrode; secondly measuring the pickup electrode potential while saidpickup electrode reference voltage is applied; calculating the chargeaccumulated on said pickup electrode using the formula:Q_(D)=E_(e)-E_(r)-(E_(i)-E_(o)) where Q_(D) is the accumulated charge,E_(e) is said pickup electrode potential immediately after allowingelectrons to impinge on to said pickup electrode, E_(r) is said pickupelectrode potential secondly measured while said pickup electrodereference voltage is applied, E_(i) is said pickup electrode potentialimmediately after disconnecting said pickup electrode reference voltagefrom said pickup electrode and E₀ is said pickup electrode referencevoltage firstly applied to said pickup electrode.
 16. The methodaccording to claim 15 wherein electrically biasing said Faraday cagefurther comprises maintaining said Faraday cage at said pickup electrodereference voltage.
 17. The method according to claim 15 whereinelectrically biasing said Faraday cage further comprises maintainingsaid Faraday cage at the same potential as said pickup electrode. 18.The method according to claim 15 wherein supporting said pickupelectrode within said Faraday cage achieves a capacitance between saidpickup electrode and said Faraday cage of less than 1 pf.